Method and apparatus for the winning of coal by the long wall support system



K. GREBE METHOD AND APPARATUS FOR THE WINNING OF COAL Feb. 10, 1970 BYTHE LONG WALL SUPPORT SYSTEM Sheets-Shet :1

Filed Jan. 31, 1968 R m M v m Feb.. 10, 1970 I K. GREBE 3,494,663

' METHOD AND APPARATUS FOR THE WINNING OF COAL BY'THE LONG WALL SUPPORTSYSTEM Filed Jan. 51, 1968 2 sheets-sheet 2 INVENTOR United StatesPatent 3,494,668 METHOD AND APPARATUS FOR THE WINNING OF COAL BY THELONG WALL SUPPORT SYSTEM Konrad Grebe, Auf dem Nutzenberg 1,Wuppertal-Elberfeld, Germany Filed Jan. 31, 1968, Ser. No. 701,922 Int.Cl. E21c 29/00 US. Cl. 299-1 11 Claims ABSTRACT OF THE DISCLOSURE Theinvention relates to a method and to an apparatus for the mechanicalwinning of coal or other minerals occuring in seam formation bylong-wall mining by means of an apparatus consisting of an advanceablesupport framework carrying interarticulated guide rail sections for alink cutter chain carrying at the face side of the chain links insections of the chain mineral removing elements including cuttingelements for producing in the coal face a channel in advance of the faceand Wedgelike shaped excavation tools in rear of said cutting toolswhich wedge in from the channel the coal, after the channel has reacheda predetermined depth so to advance the excavation face for apredetermined extent, means being provided to angularly adjust saidguide rail sections with respect to the horizontal axis, so to alter theangle of attack of the cutter chain with respect to the excavation face.

Background of the invention In the known embodiment of this method theadvance of the members of the advanceable frame work and, therefore, ofthe guides for the cutter chain arranged thereon as well as thepenetrating depth of the cutting tools, attached to the chain, into thechannel which they produce running in front of the working face arelimited by the fact that clearing tools, which are attached to themembers of the cutter chain and for their part bear the cutting members,attach themselves flatly to the working coal face. Because in this knownarrangement the thickness of the successive clearing tools which lieflatly to the working face in continuously reduced, the chain maycontinuously move nearer the face, in which case however the thicknessof the distance pieces connecting the cutting elements to the chain and,therefore, the distance between the cutting elements, which extend intothe channel, and the chain remains constant. This causes a continuous orapproximately continuous advance of the support members bearing thechain and accordingly a constant bearing of the chain on the face, i.e.not only are the cutting elements which deepen the channel brought tobear against the wall of the channel about to be chipped away by them,but the clearing tools are also continuously rubbed against the coalface outside the channel, whereby the tractive force required to movethe cutter chain and, therefore, the force capable of taking up thechain are substantially increased.

Summary of the invention These difficulties and disadvantages areeliminated by the method which forms the object of the presentinvention. In accordance with the basic concept of the invention it ispossible that not the fixed coal face, but the cutter chain guide, whichis supported on the advanceable framework and is therefore itselfstationary, forms the support for the cutting tools, successive toolsbeing arranged at increasing distances from the guide and deepen thechannel successively. The result of this new method makes it possible toreduce the forces of friction, but also at the same time to increase theprecision of the advancing operation, since the advance of the cuttterno longer depends on asimultaneous shifting of the framework which mayfor its part be subject to disturbances, and since possible unevennesson the coal face no longer influences the progress of the cutter. Whenthe cutter chain guide is in a given position, the actual depth of thecut is always clearly determined by the distance between the cuttingelements and the guide of the cutter chain, said distance being given bythe thickness of the carrier members.

The invention is therefore characterized that in a coal winning processthe cutting elements fixed on the chain successively in the direction ofmovement of the chain and their carrier elements extend from the guidein successively increasing distances in the direction of the coal face,carrier elements sliding on the chain guides which are supported on theadvanceable framework. The advanceable framework remains stationaryduring the cut and is advanced only after the passage of the activatedsection of the chain on which the cutting elements are arranged i.e.during the passage of the following section of the chain not fitted withsuch tools after the deepening of the channel produced by them for apredetermined amount, by this advance. Then introducing a wedging toolfixed on the chain following the cutting elements into the channel. Thewedging tool in advancing through the channel while the framework isstationary excavates the coal above or below as the case may be from thechannel so as to advance the coal face as such for the deepness of thechannel whereupon the next section of the chain fitted with cuttingtools supported on the then stationary framework resumes the deepeningof the channel and so on.

Therefore, after the sections of the cutter chain fitted with thecutting elements have deepened the channel by the provided measure inthe stationary support, the members of the advanceable framework can beadvanced with great precision by the action of automatically re leasedcontrol commands, each advancing frame of the frameworkcovering thepre-determined path, the distance of which is determined, for example,by using measuring cylinders which accurately establish the advance pathfor every support member. The pressure applied to execute the advancecan therefore be of any desired height, whereas in the known method itwas practically limited by the fact that the forces of friction producedwhen the distance pieces are pressed against the coal face could notexceed a specific measure as this entailed the risk of checking theprogress of the advanceable framework. In the inventive method suchirregularities can easily be avoided by making the distance between thewedging tools on the cutter chain sufliciently great and by allowing agreater margin for the time intervals in which the supporting frameworkis advanced.

As in the known method, the clearing and the wedging tools attached tothe link chain are supported on an extended flange of the rails whichguide the link chain. Since the clearing tools no longer form thedistance pieces between the cutter chain and the coal face, it becomesunnecessary for these clearing tools to be variably thick in design orfor the guide of the cutter chain to be arranged on a certain incline tothe coal face. On the contrary the guide of the cutter chain can be andis kept exactly parallel to the coal face produced by the wedging toolswhile the channel is deepened by the cutting tools. Only when thewedging tools and the following cutting tools have travelled through isthe guide brought nearer to the new coal face. Regular operation of allthe clearing tools of equal thickness is thereby ensured.

In the known method the cutting edges of the clearing tools could not beprovided with a clearance angle, because the existence of such an anglewould have resulted that the tools would have penetrated the coal facesince they and then would no longer have fulfilled their function asdistance pieces. This aspect does not apply in the, new method whichtherefore makes it possible to provide such a clearance angle so thatthe clearing tools can also cut themselves free even in the case of anirregular coal face.

Since the advanceable supports are advanced only at intervals, it issuflicient then if the frames to be advanced are braced by elasticpressure between the roof and floor only for short periods of theadvance. During cutting and wedging, the framework is set with its fullresistance pressure.

Since the guides of the chain are no longer continuously shifted andthere is no longer a constant contact between the chain or distancepieces fitted thereon and the coal face, it would be conceivable thatthe space between the chain and coal face kept clear of coal by theclearing tools is', after the latter have passed through newly breakingparts of the face, filled up before the advance is resumed so that thechannel cleared by small cuttings collapses again after the cuttingtools have travelled through. However, in order that the extractingoperation can proceed according to plan, it must be ensured that, afterthe clearingand cutting tools have passed through, the guides of thechain can be under any circumstances moved nearer to the face by theprovided measure and therefore their extended lower flanges (extendedupper flange on the roof) which each support the grooving tools can bepushed correspondingly deep into the channel. The risk that the spacerequired for advancing may be filled up again before or even during theadvance is indicated mainly in the vicinity of the floor, but ispractically non-existent in the vicinity of the roof. In order toeliminate this risk a number of preventive measures are provided inaccordance with the invention and they can be applied eitherindividually or in a combination.

In simple cases it is always suflicient if the guides of the cutterchain on the roof, in contrast to those on the floor, are rearwardlystagged with respect to the guides on the floor in the direction ofadvance of the coal winning. This has at the same time the advantagethat the caps rising above the conveyor can be shorter in design and thecoal extracted from the roof falls directly into the conveyor. Thispossibility applies just as much to the use of -a single endless cutterchain rotating parallel to the face as to the use of two cutter chains,one of which rotates parallel to the floor and the other parallel to theroof. When using a single endless cutter chain it is if need beexpedient to slightly incline the axes of the driving and reversingsprockets of this chain to the floor. However, it is also .possible tokeep these axes exactly parallel to the floor and roof and to let theconnecting guide rails of the chain on the floor run at a certain angleto the coal face and/or those on the roof run at a certain angle awayfrom the coal face.

When using two cutter chains, one of which moves in the vicinity of thefloor and the other in the vicinity of the roof, it is expedient todrive the driving wheels of both chains jointly, i.e. not to run the twochains in opposite directions but in the same direction. In this way theworking flight of the roof chain, in contrast to that of the floorchain, does not need to be rearwardly staggered, since, when flights ofchain are directly superposed, the wedging tools associated with theroof chain can be rearwardly staggered in the conveying direction of thechain in contrast to the wedging tools associated with the chain movingalong the floor. In this way the roof requires a smaller number ofclearing tools than the floor so that despite transferred wedgingdevices the clearing of the channels on the roof and floor respectivelycan be terminated simultaneously, In the case of very low seams wedgingtools can be expediently and advantageously omitted on the chain whichruns on the roof. However one should not di p se with c tt g out achannel in f ont of the co face in the vicinity of the roof, because inevery case the presence of this cutting improves the action of thewedging tools arranged on the other cuter chain. Furthermore it provideson the roof a plane for the trouble-free progress of the advanceableframework which is elastically braced between the roof and floor duringthe advance.

In seam conditions in which it is impossible to calculate that thewedging tools working on the floor operate up to the roof and viceversa, it is more advantageous to work with a single endless cutterchain which, together with all its associated tools, moves along thefloor in one direction and along the roof in the opposite direction.

In a preferred form of the invention, while advancing the guide rail,through sections of which the chain passes, its links are graduallypushed forward until they are close to the face, the enlarged flange ofthe guide rail being pressed into the channel to the same extent.Clearing tools are therefore associated with the unactivated regions ofthe link chain and they clear the part of the channel located directlyin front of the extended flange of the guide and the region directly infront of the links of the cutter chain above this flange. While theguide is pressed forward in the direction of the face by this amount,the next tool of the link chain moving along the face reaches that samepoint, said tool like-wise making space for a further advance.

In order to clean the channel several links of the cutter chain may beprovided with a shoe, extending over the width of the lower flange ofthe guide rail, said flange being extended in front of the links, andextending right down to the 'base of the channel in front of thisflange, said shoe having a shovel-like, rising surface which slides onthe lower flange of the guide and on the base of the channel, and at theforward outer end of this surface the shoe has an edge, extending overthe whole height of the channel and provided with a clearance angle, andconnected to this edge a wall, which runs up to the chain link, enclosesthe shovel-like surface and extends from the surface as far as the upperlimiting face of the channel, the upper end of said wall being in turnprovided with a cutting edge which has a clearance angle. These clearingshoes firstly clean the part of the channel directly in front of theextended flange of the guide rail and convey the coal taken up there bythe shovel-like face into the space above this extended flange. The wallenclosing the shovel-like face diverts the collected coal in thedirection of the chain. Behind this wall the coal either falls down onto the extended flange or is conveyed further upward by a tool arrangedon the chain link.

When advancing the extended flange the edge, which is arranged on theforward end of the clearing shoe and provided with a clearance angle, ispressed into the coal broken down. By means of its forward movement theclearance shoe cuts itself free and conveys the coal which it hasgrasped out of the channel. While the upper end of the wall extending asfar as the upper limiting face of the channel is in turn provided with acutting edge having angular clearance, the clearing shoe can also cutitself free along the upper limitation of the channel. When pressing theextended flange further forward the next clearing shoe can thereforepenetrate correspondingly deeper into the channel and carry on itsclearing accordingly.

Also the clearing tools associated with the activated sections of thecutter chain may be provided with clearing shoes which discharge thecoal loosened by the cutter tools out of the channel so that thefollowing cutter tools can further deepen the channel which has beencompletely cleared of the small cuttings. These clearing shoes connectedto the cutting tools may be similar in design to the usual clearingshoes described, only their parts arranged above the extended flange ofthe guide rail being replaced by the corresponding parts of the clearingtools with which they are associated, and

their parts arranged in front of the extended flange of the guide railnot only extend over a narrow strip of the channel, but also extend overits entire width. While the normal clearing shoes are all of the samewidth, the clearing shoes associated with the activated sections of thecutter chain become successively wider by the same amount by which thecutting elements associated with the preceding clearing tool havedeepened the channel. The space between the links of the cutter chainand the coal face, i.e. the space above the extended flange of the guiderail which is kept clear in the activated sections of the cutter chainby the clearing tools, after the grooving tool connected to the latterhas advanced the coal face, does not need to be kept constantly clear,but the bulk material which, after the activated sections of the cutterchain have travelled through, may have accumulated in the space,requires to be discharged to a degree such as this space is constrictedby the further advancing of the cutter chain and its guides.

For this purpose some of the chain links not fitted with wedging orcutting tools may be provided with a narrow edge which rises in the formof a ramp from the lower flange of the guide rail, rearward over thewhole length of the link and projects with angular clearance towards thecoal face. The links are of normal thickness only below this edge,whereas above the edge there is a recess. This recess provides a ramprunning in the direction of the edge, on which ramp the bulk material,gripped by the protruding edge cuts itself free during the advance ofthe chain, is guided and conveyed upwards.

With respect to the face of the links on the coal face side, the upperflange of the guide rail is displaced rearward for the width of therecess of the link arranged above the rising edge, so that two linksprovided with the rising edge may each follow a further link which, inthe region before the upper flange of the guide rail, is provided withan extension which prolongs the ramp at least as far as the upper edgeof the guide rail, said ramp like-wise having an edge projecting withangular clearance towards the face.

The successive advancing of the support framework is expedientlycontrolled by control members which are themselves associated with thecutter chain running through the guide rail.

Since, in the case of a chain endless and rotating in the plane of theface, the cutter chain covers half of its path along the floor where thedanger of the control operations, being disturbed by foreign bodies isparticularly great, a control chain rotating practically synchronouslywith the cutter chain can for example be used advantageously forgenerating control impulses, the lower flight of which chain can beguided at any desired distance from the floor. The drive of this controlchain is derived from the drive or drives of the cutter chain, in such amanner that corresponding links of both chains are always located in thesame region of the long wall face. The idea of using a control chainwhich, moving along the face synchronously with the motion of anextracting device, controls the advance of the support framework or ofthe conveyor corresponding to the progress of the extracting operation,can also be applied to other winning devices moved, such as coal-ploughsnormal cutting machines and the like, and as such is therefore anindependent feature of the invention.

An advancing cylinder may be employed, acting on the associated floor orroof cap of the advance support framework and supported on a stationaryframe of the support framework, and associated with every section of theguide rail in which the cutter chain moves, and preferably supplied witha quantity of pressure fluid by the actuation of a measuring or meteringcylinder connected thereto, said quantity of pressure fluid causing theguide rail to be advanced by the prescribed amount. Each measuringcylinder is actuated by shifting an associated valve,

whereby either the left chamber of the measuring cylinder is, forexample, filled from a source of pressure fluid and at the same time theright chamber empties into the pressure cylinder or, after the shiftingof the valve, the now filled left chamber empties into the pressurecylinder for advancing and the right chamber is filled from the pressurefluid source. The said valves may be shifted one after another by acontrol member which moves past them and is connected to each activatedsection of the cutter chain provided with extracting tools or to thecorresponding section of the control chain.

Brief description of the drawings FIGURE 1 shows a purely plan viewschematic representation of a working illustrating the method,particularly the course of the guide shifting operation, and thedimensions in the longitudinal direction of the long wall face arerepresented as being considerably shortened in order to make it possibleto show the gradual advance of the working face and the advance of theentire arrangement in as great an area of the long wall face aspossible.

FIGURES 2 to 5 are cross-sections through the extracting apparatus insuccessive phases of the extracting operation according to the lines11-11, IIIIII, IV-IV and VV of FIGURE 1.

FIGURE 6 shows a section of the chain occupied by the extracting toolsas seen from the coal face.

FIGURE 7 is a top view of FIGURE 6 with the omission of the upper flangeof the guide rail, and

FIGURE 8 shows a section through an outer strap of the chain along theline VIIIVIII of FIGURE 6.

Description of the preferred embodiment of the invention In the drawingsthe coal face is denoted by 1, the channel produced therein by 2, andthe rail of generally U-shape guiding the cuter chain by 3, said guiderail being provided with a forward edge 4 on its upper flange whichoverlaps the cutter chain guided on the floor and with a forward edge 5on its extended lower flange, which forward edge ends on the coal face,but, during the advance and then until the new face is produced, it ispushed into the channel 2 cut into the coal face.

The cutter chain itself consists of outer links 6, 6' and inner links 7.The link 6' is larger in design than the link 6 and is guided throughgrooves between the rail flanges in order to prevent the chain frommoving out of place. The members of the chain and the tools arrangedthereon are not shown in FIGURE 1.

The outer links 6 are fastened in the activated sections of the cutterchain to a multipart wedging or grooving tool 8 (FIGURE 2) which isfollowed by several multipart clearing tools 9 (FIGURE 3) which haveessentially the same profile. The wedging or grooving tools 8 are of thesame design shown in my US. Patent 3,390,915. The successively arrangedclearing tools are all of the same thickness and the knife 9 associatedtherewith has a clearance angle projecting in the direction of the coalface. The forward edge of this knife lies close to the coal face, but isnot intended to penetrate into the face in the normal manner and canonly cut itself free in the event of unevenness of the coal face, orduring controlled movements of the rail for the purpose of altering thecutting plane. Therefore the faces of the clearing tools normally do notabut the coal face. Cutter heads 10 for deepening the channel areattached to the clearing tools. Successive cutter heads associated withthe same clearing tool have cutting tools 11 operating at differentlevels. Thus another cutter head is moved in front of the cutter headshown in FIGURE 3, the cutting tool of the first cutter head havingoperated at a medium level of the channel, and is followed by a furthercutter head, the cutting tool of which correspondingly deepens thechannel also in its uppermost area. The next clearing tool is providedwith a clearing shoe (not shown) which extends over the whole width ofthe channel and clears it of small cuttings. This clearing shoe in turnis followed by a set of cutting tools which penetrate deeper into thechannel. The effectiveness of seven successive sets of such cuttingtools can be seen in FIGURE 1 at the right end and between the sectionsIIII and IIIIII.

After the activated section of the cutter chain has advanced anddeepened the channel, there then follows an unactivated section of thecutter chain designed according to FIGURES 4 to 8 at a certain distancefrom the face, which corresponds to the thickness of the clearing tools9 (FIGURE 4) and after the advance into contact with the coal face(FIGURE To some of the links 6 of the chain, a clearing shoe 12 iscoordinated. This clearing shoe has a shovel-like face 13 sliding on thebottom of the channel and a parallel surface 13 sliding over theextended flange 5 of the guide rail. At the forward outer end of theshovel-like surface 13 is formed an edge 14 which extends over theentire height of the channel, the edge 14 being followed by an edge 15running along the upper end of a wall 15 which encloses the shovel-likeface and is provided with a support 25. The edge 15 provided with anangular clearance just like the edge 14 can be prolongated by a ramp 16which is concurrently arranged to the rising face 13, 13' and extendsacross the entire height of the channel. This ramp can be seen moreclearly in FIGURES 6 to 8 on the next link which is not provided with aclearing shoe. It is formed by providing the link 6 with a rising edge17 projecting with angular clearance towards the coal face, said linkbeing of normal thickness only in its part 26 located in front of thisedge in the travelling direction of the chain and in the part 26'located below and behind this edge, while above the edge 17 the link hasa recess, as can be seen particularly in FIGURE 8.

Opposite the upper flange 4 of the guide rail the links of the chainproject slightly towards the coal face (approximately by the width ofthe ramp 16). The edge 17 forms another projection which, after theguide is advanced somewhat, which abuts against the new coal faceproduced by the wedging or grooving tool 8. This edge 17 grasps andconveys away coal which eventually could have accumulated after thepassage of the wedging tool 8 and clearing tools 9 following the tool 8between the chain and the coal face. The remaining sections of the chainlinks do not come into direct contact with the face. In order to preventthe falling down or jamming of the coal conveyed upwards over the ramp16 between the extension 4' of the guide rail and the coal face, theinner link 7 of the chain which follows the outer link 6 provided with aramp 16 can be provided with an upwardly directed extension 18 which hasa ramp 19 which prolongates the ramp 16 at least as far as the upperedge of the extension 4' the ramp 19 being for its part provided with anedge 20 projecting towards the coal face.

In operation corresponding to the progress of the cutter chain, theschematic representation in FIGURE 1 shifts to the right with theprogress of the wedging and clearing tools, the advance of the coal faceand the progress of the deepening of the channel the advance of theadvanceable support framework with the guide rails supported by itprogresses one by one in automatically controlled manner. The frameswhich were in advanced position are then in rear at the end of theadvance of the following frame, the oblique part of the guide rails seenin the middle of FIG. 1 being shifted to the right parallel to itselfapproximately at the same pace as the cutter chain.

There is incorporated in this application by reference applicants priorUS. Patent 3,390,915, dated July 2, 1968, showing and describing thesupport framework for the guide rails which in turn receive the cutterchain for the tools. Thus, such structure as well as the mechanism foradvancing same, is incorporated by reference in this application.

What I claim is:

1. A method for the mechanical extraction of coal and other minerals bylongwall mining by means of an apparatus consisting of an advanceablesupport framework carrying interarticulated guide rail sections for alink cutter chain with means for angularly adjusting said guide railsections with respect to the horizontal axis to alter the angle ofattack of said cutter chain with respect to the excavation face,wedge-like mineral removing elements carried at the face side of linksor groups of links of the chain and cutting elements for producing achannel in advance of the coal face arranged on other links or groups oflinks as well as clearing tools fixed on the cutter chain, said chainhaving sections in which such tools are absent, the wedgelike elementsexcavating the coal from the channel supporting themselves in thechannel, after the latter has reached a predetermined depth foradvancing the excavation face a predetermined extent, characterized inthat the cutting elements fixed on the chain following one another inthe direction of movement of the chain respectively, carrier elementsfor said cutting elements extending from the guide in successivelyincreasing distances in the direction of the coal face and sliding onthe chain guides supported on the advanceable framework, said frameworkremaining stationary during the cutting out of the channel, the guidesfor the chain together with the framework being advanced for apredetermined distance after the passage of a section of the chainfitted with cutting tools and by this advance introducing a wedging toolfixed on the chain following the cutting elements into the channel forperforming the excavating phase to advance the coal face as such for thedecpness of the channel-whereupon the next section of the chain fittedwith cutting tools supported on the now again stationary frameworkresumes the deepening of the channel and so on.

2. A method as claimed in claim 1 in which a channel is simultaneouslyproduced in the roof and in the floor region of the coal face,characterized in that the channel in the roof area is staggered inrearward direction with respect to the channel produced in the floorarea by correspondingly staggering the guiding rails of the chain.

3. A method according to claim 1 characterized therein that the advanceof the supporting framework is automatically controlled in dependencyupon the movements of the chain.

4. An apparatus for mechanically winning of coal or other minerals bylongwall comprising at least one chain running in the area of the roofand of the floor and guided in angularly adjustable rail sections withrespect to the horizontal axis which sections are attached to theframework of an advanceable support framework, said chain comprisingsections in which channel cutting tools are attached to the chainpreceded by wedge-like acting excavating tools and sections of the chainin which tools are absent, the cutting tools being attached to the chainby carrier members in successively increasing distances from the chain.

5. Apparatus as claimed in claim 4 characterized therein, that thecarrier members form themselves clearing tools for the channel and forthe space between the coal face and chain.

6. Apparatus as claimed in claim 4 comprising a control chain associatedwith the cutter chain running prac tically synchronously therewith andserving to generate impulses which control the advance of the frameworksupporting the guides of the chain.

7. Apparatus as claimed in claim 4 characterized therein, that as wellin the area of the roof as in the area of the floor a chain runs in theplane of the seam and arranged parallel to the floor and the roof,driving sprockets for both of the chains, and a common drive for saidsprockets.

8. Apparatus as claimed in claim 7, in which the guides of the cutterchain on the roof are arranged to pivot oppositely to those of thecutter chain on the floor.

9. Apparatus as claimed in claim 4 comprising U shaped chain guidesarranged with the web in the plane of the coal face, the guides actingas a tool carrier, at least some of the chain links of the saidintermediate chain sections having edges which rise in the form of rampsfrom the lower flange of the guide diagonally and backwards over theWhole height of the link and project with angular clearance towards thecoal face, said links being of their normal thickness only below theedges, but having recesses above the edges.

10. Apparatus as claimed in claim 9, wherein the forward edge of theupper flange of the guide opposite the face of the links on the coalface side is reversed by the width of the recess of the links arrangedabove the rising edge,

11. Apparatus as claimed in claim 9, wherein several links, which areprovided with a ramp formed by the rising edge and the recess of thelinks, are followed by another link having an extension ramp edgecontinuing 10 the preceding rising ramp edge at least as far as theupper edge of the guide rail, the ramp of the extension having an edgewhich projects with angular clearance towards the face.

References Cited UNITED STATES PATENTS 3,390,915 7/1968 Grebe 29932FOREIGN PATENTS 1,21 ,019 3/1966 Germany.

706,026 3/1954 Great Britain.

ERNEST R. PURSER, Primary Examiner US. Cl. X.R. 299-18, 32, 34

